We launched a fully revamped Sign-In API with Google Play services 8.3 providing a much more streamlined user
experience and enabling easy server authentication and authorization. We’ve heard from many developers that they’ve found these APIs simple and
less error prone to use. But when we look at applications in the Play Store, we
see many that are still using legacy Plus.API / GoogleAuthUtil.getToken and do
not follow best practices for authentication and authorization. Not following best practices can make your apps easily vulnerable to attack.
It’s also worth noting that developers who don’t want to worry about managing the security implications of different API flows or keeping up to date with the latest APIs can use Firebase Authentication to manage the entire authentication lifecycle.

Ensuring your apps are secure

Developers should ensure that their apps are not open to the following
vulnerabilities:

Email or user ID substitution attack
After signing in with Google on Android, some apps directly send email or
user ID in plain text to their backend server as the identity credential. These
server endpoints enable a malicious attacker to easily forge a request and gain
access to any user’s account by guessing their email / user ID.

Figure 1. Email / User ID Substitution Attack
We see a number of developers implement this anti-pattern by using getAccountName or getId from the Plus.API and sending it to their backend.Problematic implementations, DO NOT COPY

Access token substitution attack
After signing in with Google on Android, many apps send an access token
obtained via GoogleAuthUtil.getToken to their backend server as the identity
assertion. Access tokens are bearer tokens and backend servers cannot easily
check if the token is issued to them. A malicious attacker can phish the user
to sign-in on another application and use that different access token to forge
a request to your backend.

Figure 2. Access Token Substitution AttackMany developers implement this anti-pattern by using GoogleAuthUtil to retrieve
an access token and sending it to their server to authenticate like in the
following example:

If your server needs access to other Google services, e.g. Drive, you should
use server auth code flow. You can also check out this blogpost. Worth mentioning, you can also get an ID token using server auth code flow,
from which you can retrieve user id / email / name / profile url without
additional network call. Check out the migration guide.

Improving the user experience and performance of your apps

There are still many apps using GoogleAuthUtil for server auth and their users
are losing out the improved user experience while the developers of those apps
need to maintain a significantly more complicated implementation.
Here are some of the common problems that we see:

Many apps request GET_ACCOUNTS permission and draw their own customized picker
with all email addresses. After getting the email address, the app calls either
GoogleAuthUtil or Plus.API to do OAuth consent for basic sign in. For those
apps, users will see redundant user experience like:

The worst thing is the GET_ACCOUNTS permission. On Marshmallow and above, this
permission is displayed to the user as ‘Contacts’. Many users are unwilling to
grant access to this runtime permission.

Solution

Switch to our new Auth.GOOGLE_SIGN_IN_API for a streamlined user consent experience by providing an intuitive one-tap
interface to provide your app with the user’s name, email address and profile
picture. Your app receives an OAuth grant when the user selects an account,
making it easier to sign the user in on other devices. Learn more

Figure 4. New streamlined, one-tap sign-in experience

Getting ID Token from GoogleAuthUtil for your backend

Before we released revamped Sign-In API, GoogleAuthUtil.getToken was the previously recommended way to retrieve an ID
token via a “magic string.”

Wrong pattern, DO NOT COPY

GoogleAuthUtil.getToken needs to take an email address, which usually leads to
the undesirable user experience in Figure 3. Also, user’s profile information
like name, profile picture url is valuable information to store in your server.
The ID token obtained via Auth.GOOGLE_SIGN_IN_API will contain profile information and your server won’t need additional network
calls to retrieve them.
Solution
Switch to the ID token flow using the new Auth.GOOGLE_SIGN_IN_API and get the one-tap experience. You can also check out this blogpost and the migration guide for more details.

Getting auth code from GoogleAuthUtil for your backend

We once recommended using GoogleAuthUtil.getToken to retrieve a server auth
code via another “magic string.”

Wrong pattern, DO NOT COPY

In addition to the possible redundant user experience in Figure 3, another
problem with this implementation was that if a user had signed in to your
application in the past and then switched to a new device, they would likely
see this confusing dialog:Figure 5. Confusing consent dialog for returned user if using
GoogleAuthUtil.getToken for auth codeSolution

Should I ever use GoogleAuthUtil.getToken?

In general, you should NOT use GoogleAuthUtil.getToken, unless you are making
REST API call on Android client. Use Auth.GOOGLE_SIGN_IN_API instead. Whenever possible, use native Android API in Google Play services
SDK. You can check out those APIs at Google APIs for Android.
And starting from Google Play services SDK 9.0, you will need to include -auth SDK split to use GoogleAuthUtil.getToken and related classes
AccountChangeEvent/AccountChangeEventsRequest/AccountChangeEventsResponse.dependencies {
compile 'com.google.android.gms:play-services-auth:9.0.0'
}